The present invention relates to the use of vehicle suspension systems so as to control the steering characteristics of the vehicle.
Under harsh steering conditions many vehicles exhibit a certain degree of either under-steer, where the vehicle does not corner as tightly as the angle of the steered wheels suggests it should, or over-steer, where the vehicle tends to spin round too quickly under cornering. Under-steer is caused by loss of grip between the front, steered, wheels of the vehicle and the ground, which prevents them from steering the front of the vehicle round the corner, and over-steer is caused by loss of grip between the rear wheels and the ground, which allows the rear of the vehicle to swing outwards when cornering. The present invention aims to provide a system in which these problems can be reduced by controlling more accurately the level of traction or grip between the front and rear wheels and the ground during corning.
It is known from U.S. Pat. Nos. 4,974,875 and 4,903,983 to provide a suspension in which the load can be transferred between wheels so as to control the grip of each wheel and hence the cornering characteristics. Another such system is disclosed in DE 38 17 540.
Accordingly the present invention provides a vehicle suspension system comprising an actuator arranged to produce an actuator force between a sprung part of the vehicle and a part of the suspension which can move relative to said sprung part thereby to allow vertical travel of a wheel, measuring means arranged to measure cornering of the vehicle, and control means, characterized in that the control means is arranged, in the event of predetermined cornering conditions, to promote lateral slipping of the wheel by producing a rapid variation in said actuator force so as to produce a pulsing of the contact force between the wheel and the ground causing a reduction in adhesion of the wheel, so as to control the steering characteristics of the vehicle.
The pulsed variation may be arranged to promote lateral slipping of the front wheels of the vehicle during cornering thereby reducing over-steer. Conversely it could be arranged to promote lateral slipping of the rear wheels of the vehicle during cornering thereby reducing under-steer.
Preferably the measuring means is arranged to measure lateral acceleration of the vehicle, and the control means is arranged to control the actuator force in response to the measured lateral acceleration
In one embodiment the control means is arranged to produce a sharp change in actuator force when the measured lateral acceleration reaches a predetermined level such that a change in lateral acceleration of the vehicle produces a pulse in the actuator force. This will induce vertical vibration of the relevant wheel mounted on said suspension system relative to the sprung part of the vehicle at its natural frequency.
In another embodiment the control means is arranged to produce pulsed variations in the actuator force of a controlled frequency in the event of said predetermined cornering conditions. This will produce vibration of the relevant wheel at that controlled frequency.
The present invention can be applied to any form of active suspension. Accordingly the actuator may comprises a strut arranged to produce forces in a vertical direction between a wheel of the vehicle and a sprung part of the vehicle, as in a fully active gas suspension system, for example. Alternatively the actuator may be arranged to urge two wheels of the vehicle in opposite vertical directions, as in an active roll-control system.